Carburettor

ABSTRACT

A mixture supply control system for an internal combustion engine of a motor vehicle, comprising valve means for controlling the effective area of the main fuel jet of the main mixture circuit, and temperature sensing means for controlling the operation of the valve means in response to the temperature of the engine. The mixture supply control system supplies to the engine an appropriate amount of enriched mixture for the initial cold engine and subsequent warming-up operations so as to improve the drivability of the vehicle.

United States Patent Masakiet al.

[451 Sept. 26, 1972 [s41 CARBURETTOR [72] Inventors: Kenji Masaki, Yokosuka; Sinzo Kato, Tokyo, both of Japan [73] Assignee: Nissan Motor Company, Ltd., Yokohama,Japan 22 Filed: Sept. 3, 1970 21' Appl. No.: 69,392 30] Foreign Application Priority Data Sept. 9, 1969 Japan ..44/71375 [52] US. Cl. ..261/39 B, 261/39 D, 261/121 A, 261/41 D, 26l/D1G. 38, 123/180 E, 236/102, 251/D1G. 4

[51] Int. Cl ..F02m 1/10 [58] Field of Search ..261/39 D, 39 A, 39 B, 41 D; 123/180 E; 236/102 [56] References Cited UNITED STATES PATENTS 2,074,471 3/1937 Holley et a1 ..261/39 D 3,278,171 10/1966 Carlson ..261/39 D 1,544,350 6/1925 Sisson ..261/39 D 2,711,883 6/1955 Reeves ..261/41 D Briggs ..261/39 D 6/1946 Hunt ..123/180 E 3,374,951 3/1968 Jackson et al. ..236/102 2,341,694 2/1944 Coffey ..261/D1G. 38 3,587,553 6/1971 Sutton ..261/39 D 1,699,468 1/1929 Grayson ..236/102 3,447,746 6/1969 Visos ..236/102 FOREIGN PATENTS OR APPLICATIONS 1,337,410 4/1963 France ..261/D1G. 19

Primary Examiner-Tim R. Miles Attorney-McCarthy, Depaoli, OBrien and Price [5 7] ABSTRACT A mixture supply control system for an internal combustion engine of a motor vehicle, comprising valve means for controlling the effective area of the main fuel jet of the main mixture circuit, and temperature sensing means for controlling the operation of the valve means in response to the temperature of the engine. The mixture supply control system supplies to 'the engine an appropriate amount of enriched mixture for the initial cold engine and subsequent warming-up operations so as to improve the drivability of the vehicle.

2 Claims, 2 Drawing Figures M iXTURE PATENTEU SEPZB I97? 3. 693. 947

sum 1 [1F 2 ATTORNEYS CARBURETTOR This invention relates to carburettors for internal combustion engines used in motor vehicles and particularly to carburettors having a mixture supply control system for feeding to the engine a satisfactory amount of enriched air-fuel mixture for the initial cold-engine and subsequent warm-up operations.

Nowadays, air pollution is a serious social problem. Although there are a number of sources of noxious air pollutants, the pollutants emitted in engine exhaust gases are increasingly prominent. With a view to minimizing such emissions, various vehicular air-pollution preventive methods have been proposed including the improvements of the engine and/or carburettor performance.

As is well known in the art, the engine exhaust emission of the noxious air pollutants such as carbon monoxide, hydrocarbons and nitrogen oxides can be reduced to a minimum only if an air-fuel mixture is burned completely at its stoichiometric ratio in the combustion chamber. It is found, however, quite difficult to accomplish complete combustion in the engine for all the varying drive conditions of the vehicle. As a matter of fact, the above noxious pollutants are produced in different quantities varying in the kind of the pollutants dependent on the running conditions of the engine. For instance, the quantity of nitrogen oxides emitted is large when the air-fuel mixture is lean bons are large when the mixture is rich. The emission of nitrogen oxides can be made so small, however, as to cause no serious air-pollution problem even with a lean mixture, if the engine has not fully warmed up. Under these circumstances, there has been a growing tendency to construct the carburettors of known types in a manner to provide a lean mixture without or with little respect tothe varying driving conditions of the vehicle.

This tendency to provide a lean mixture inevitably invites deterioration of drivability of the vehicle due to the degradation of operational performance of the engine. T he drivability deterioration is often experienced as such phenomena as engine stalling when starting, lack of power for acceleration or knocking of the engine under all driving conditions from low to intermediate speed ranges of the vehicle, and occurs particularly when the ambient temperature is low and/or when the engine is being warmed up. In addition, socalled cold driving" is often performed these days by most drivers without allowing ample time for warmingup, thus affecting the drivability of the vehicle still more.

In general, gasoline as usually used for fuel of the motor vehicle does not satisfy all of the engine operating requirements during the cold driving because of its lack of volatility. In order to acquire a stable drivability of the vehicle, an appropriate amount of enriched airfuel mixture is needed especially for cold driving. As has been discussed, however, existing carburettors are set to provide a lean mixture so as to reduce the emission of the noxious air pollutants, thus inviting deterioration in drivability. This deterioration is encountered in existing carburettors, since when carburetor operation is changed from slow-running mixture circuit to the main mixture circuit little or no mixture is supplied to the intake manifold for the transitional operation.

while the quantities of carbon monoxide and hydrocar- In order to overcome these drawbacks, there has been proposed a method in which the mixture flow rate supplied through the two circuits is increased for this particular driving condition. This method aids in improvement of the. drivability but a large amount of the air pollutants are emitted into the open air from the exhaust pipe. Nowadays, however, this method can not be employed under the regulations of the law.

This invention is based on the experiments conducted by us using carburettors which provide a lean air-fuel mixture, which experiments have revealed that during the cold driving" the improvement of drivability is effected if an enriched mixture is supplied through the main mixture circuit rather than through the slowrunning circuit. This means that an enriched air-fuel mixture is fed to the engine for the initial cold-engine and subsequent warm-up operations. These preparations necessary for warming up the engine up to a predetermined level before starting to move the vehicle are known to continue for 5 to 15 minutes depending upon the ambient temperature. The temperature level thus determined previously is, for example, 50 C. in terms of the temperature of the engine cooling liquid. Considering the short period ranging from 5 to 15 minutes, the emission of the pollutants contained in the exhaust gases during the cold driving is not considerable when compared with the total amount exhausted during all driving conditions, even if an over-rich mixture is consumed in the engine during this period.

The mixture supply control system proposed by this invention is constructed and arranged in such a manner that an air-fuel mixture having a sufficiently rich airfuel ratio is fed to the engine during the initial cold-engine and subsequent warm-up operations by varying the effective area of the main fuel jet. This effective area is controlled by valve means provided in the neighborhood of the main jet in response to the temperature of the engine which condition is detected by temperature sensing-means.

Thus, it is an object of the invention to provide a carburettor of a type adapted to feed to the engine an enriched mixture during the initial cold driving of the vehicle and to feed a usual lean mixture as soon as the engine has been sufficientlyv warmed up.

Another object is to provide a carburettor having a main fuel jet of varying effective area and valve means for controlling the effective area of the main fuel jet responsive to the temperature condition of the engine, thereby improving the drivability of the vehicle.

In the drawings:

FIG. 1 is a schematic sectional overall view of a construction of the carburettor having a mixture supply control system according to the invention, in which the effective area of the main fuel jet is controlled by a solenoid valve assembly responsive to the temperature of the engine; and

FIG. 2 is similar to FIG. 1 but shows a modification of the system, in which the effective area of the main fuel jet is continuously controlled by a bimetal valve assembly.

Throughout the accompanying drawings, like characters and reference numerals will designate corresponding parts or elements. The mixture supply control systems implementing the invention, which are generally denoted by numerals l0 and 10', respectively, in FIGS. 1 and 2, are used in a conventional carburettor 12. The carburettor 12 is, as is well known, provided with a throttle valve 14, a main venturi 16, a small venturi 16, a main mixture circuit 18, an idling and slow running mixture circuit 20 and float chamber 22. The throttle valve 14 is mounted on a rotatable shaft 14a in the carburetor induction passage and is herein shown as substantially fully closed to effect idling or deceleration of the motor vehicle. The main mixture circuit 18, through which an air-fuel mixture is supplied to the carburetor induction passage for relatively heavy load operation such as acceleration or high speed driving of the engine, opened into the small venturi 16'; The idling and slow-running mixture circuit 20 for idle or light load operation opens to the carburetor induction passage downstream of the venturi 16 through a slow-running port 24 and idling port 26. The slow-running port 24 is located at a position closely adjacent to the periphery of the throttle valve 14 when it is substantially closed, while the idling port 26 is located posterior to or downstream of the throttle valve 14. Designated by numeral 28 is an idling adjustment screw for adjusting the flow rate of the mixture through the idling port 26.

There is provided in the main mixture circuit 18, a main fuel jet 30, a main fuel and air mixer 32 and a main nozzle 34 in this sequence from the float chamber 22. The main atomizer 32 has formed at its bottom an orifice 32a and at its top an air bleed 32b vented from the open air. The orifice 32a and air bleed 32b are so calibrated as to provide a desired amount of lean airfuel mixture, so that a lean mixture is produced through the main-circuit 18 for delivering to the engine so as to minimize the air pollutant content in the engine exhaust gases. In operation, with the throttle valve 14 substantially fully opened fora relatively heavy load operation, a sufficient vacuum is established in the small venturi 16' due to the fact that a considerable amount of air is passing through the small venturi 16 during this particular operation. Then, a desired amount of fuel which is metered by the main fuel jet 30 is mixed with air in the main mixer 32 to provide an airfuel mixture, which is drawn through the main nozzle 34 into the small venturi 16' by the vacuum established therein.

In addition to the slow-running port 24 and idling port 26 there is provided in the idling and slow-running mixture circuit 20 a slow-running and air mixer 36 which has formed at its bottom an orifice 36a and a first air bleed 36b vented from the ambient atmosphere. Downstream of the first air bleed 36b is provided a second air bleed 36c which is also vented from the ambient atmosphere. The first and second air bleeds 36b and 360, respectively, are so calibrated as to admit air from the ambient air at desired flow rates. While the engine is being driven under idling or light load conditions with the throttle valve 14 substantially fully closed, the air flow rate delivered into the engine is not large and, therefore, a high vacuum is not established in the small venturi 16'. Thus, a metered air-fuel mixture is supplied to the engine through the slow-running port 24 and/or idling port 26. 7

According to this invention, there is provided in the conventional carburettor described above a mixture 1 supply control system which comprises in combination a valve assembly 38 for controlling the effective area of the main jet of the main fuel mixture circuit 18 and temperature sensing. means 40 for controlling the operation of said valve means 38 in response to the temperature of the engine.

The valve assembly 38 implementing this invention may be of any type such as of solenoid or bimetal type, inasmuch as it can be actuated to control the effective area of the main fuel jet 30 in response to the temperature of the engine. The mixture supply control systems wand 10 of the invention employing the two different type valve assemblies mentioned above are shown in FIGS. 1 and 2. The temperature sensing means 40 functions to detect the temperature of the engine. This temperature detection may be carried out in this invention by means of a bimetal device, which may be located in the neighborhood of the carburettor 12 or immersed in the engine cooling liquid. The temperature sensing means 40 used is adapted to control mechanically or electrically the operation of the valve assembly 38 when the temperature sensed is below a predetermined value, namely, when the engine is running cold.

An example of a mixture supply control system to which this invention is directed is illustrated in FIG. 1. In this embodiment, the control system designated by numeral 10 has a valve assembly 38 of solenoid type and the temperature sensing means 40 is a thermoswitch 42. The solenoid valve assembly 38 has a needle element 380 which is axially movable upon being actuated by a solenoid device 44 including a solenoid coil 44a, an axially movable core 44b coaxially integral with the needle element 38a and a compression spring 44c seated on the inside of one endwall of the stationary body of the valve assembly 38 for biasing the movable core 44b toward the orifice of the main fuel jet 30. The solenoid'coil 44a is electrically connected in series to a power source 46 through a main switch 48 and the thermoswitch 42 by a line 50. The thermoswitch 42 is constituted, for instance, as a normallyopen relay switch which is connected to and actuated by a temperature detector (not shown). This temperature detector is adapted to detect the temperature of the engine, so that it may be provided directly on the wall or in the air cleaner (not shown) of the carburettor 12 or may be immersed in the engine cooling liquid.

It should be appreciated as one of the important features of the valve assembly 38 according to the invention that the needle element 380 is provided at its operative end with a conical tip 38b having a conduit 38c extending therethrough. The effective area of this conduit 380 is calibrated as to allow a predetermined amount of the liquid fuel to flow from the float chamber 22 into the main mixture circuit 18, when the conical tip 38b of the needle element 38a is in abutting engagement with the main fuel jet 30. The flow rate of the fuel through the conduit 38c is such that the main mixture circuit supplies a satisfactory amount of lean mixture to the engine after the engine has reached its normal operating temperature.

In operation, the thermoswitch 42 is closed when the temperature detector detects a cold engine and/or warming-up of the engine. When the solenoid coil 44a is energized with the thermoswitch 42 closed, the movable core 44b is retracted away from the main fuel jet 30 together with the needle element 38a against the action of the compression spring Me. This unseats the needle tip 38b 38a from the orifice of the main fuel jet 30, thus providing increased communication between the main fuel circuit 18 and the float chamber 22. As a result, a rich mixture is produced by the main mixture circuit 18. When the solenoid coil 44a is de-energized after the engine has reached its normal operating temperature with the thermoswitch 42 opened, the movable core 44b is urged toward the main fuel jet 30 together with the needle element 38a by the action of the compression spring 440. Then, the conical tip 38b of the needle element 38a is seated on the main fuel jet 30, thus providing communication between the main mixture circuit 18 and the float chamber 22 only through the conduit 380. It follows that a predetermined amount of the liquid fuel is supplied into the main mixture circuit 18, thus providing a lean mixture.

Referring now to FIG. 2, there is provided a bimetal operated valve assembly 38' for regulating the effective area for fuel flow. The bimetal valve assembly 38 has also a needle element 38a similar in construction and operation to the needle element 38a of the former embodiment. Therefore, the needle element is also provided at its operative end with a conical tip 38% having a conduit 38'c extending therethrough. This valve assembly 38' further comprises in combination a spring seat plate 52 secured to the end of the needle element 38 'a opposite thetip 38'b, a compression spring 54 engaging the inside of one end wall of the stationary body of the valve assembly 38' for biasing the plate 52 toward the main fuel jet 30 together with the needle element 38'a. It should be appreciated as a feature of this embodiment that the operation of the valve assembly 38' is controlled by a bimetal device 56 acting as the temperature sensing means 40. The bimetal device 56 is positioned between and in abutting engagement both with the plate 52 and with the other inside end wall of the body of the valve assembly 38. The bimetal device 56 has at least one bimetal unit 56a having a central aperture (not numbered) through which the needle element 38a is axially movable toward and away from the main fuel jet 30. In FIG. 2, three bimetal units 56a are shown in axially aligned series, merely by way of example. Each bimetal unit 56a consists of an annular disc 56b and a curved bimetal member 560 and secured at its ends to the disc 56b. The bimetal member 560 is adapted to expand as the ambient temperature is lowered and to contact as the ambient temperature is raised. This ambient temperature indicates the temperature of the engine, as has been discussed.

As the ambient temperature, in operation, rises, the curvature of the bimetal member 56c is reduced so that the axial length of the bimetal 56c is reduced. As the ambient temperature falls, the bimetal unit 56c increases in axial length. It should be noted here that the valve assembly 38' using this bimetal device 56 can vary continuously the effective area or clearance formed between the surface of the conical tip 38'b and the main fuel jet 30. As a result, the fuel flow rate passing through this effective area is continuously controlled in accordance with the ambient temperature.

It will now be appreciated from the foregoing description that, according to one important aspect of the invention, a sufficient amount of enriched mixture is supplied to the engine during the cold running of the engine, whereby the engine can be driven with a satisfactory quality performance as well as with a quick warm-up period. According to another important aspect of the invention, lean mixture is supplied to the engine when has reached its normal operating temperature. With these in mind, the mixture supply control system of the invention can reduce significantly the overall air pollutant emission contained in the engine exhaust gases and improve the drivability of the vehicle.

What is claimed is:

i. in a carburetor for an internal combustion engine having a float chamber, a carburetor induction passage, a throttle valve disposed in said carburetor induction passage, a main venturi, and a small venturi, the combination comprising a main mixture circuit opening into said small venturi for supplying an air-fuel mixture to said induction passage for high-speed and acceleration operations, of said engine, said main mixture circuit having a main fuel jet communicating with said float chamber, an idling and slow-running mixture circuit opening into said induction passage downstream of said main venturi for supplying an air-fuel mixture to said induction passage for low-speed and deceleration operations of said engines, said idling and slow-running mixture circuit having a slow-running port located at a position closely adjacent the periphery of said throttle valve when it is substantially fully closed and'an idling port located downstream of said throttle valve, a solenoid valve assembly for controlling the effective area of said main fuel jet of said main mixture circuit, said solenoid valve assembly including a needle element which is axially movable into abutting engagement with said main fuel jet and which is provided at its operative end with a conical tip having a conduit extending therethrough, the efiective area of said conduit being predetermined so that a liquid fuel may flow from said float chamber into said main mixture circuit when the conical tip of said needle element is in abutting engagement with the main fuel jet thereby to provide a satisfactory amount of lean mixture, said solenoid valve assembly further including a solenoid coil electrically connected in series with a power source, an axially movable solenoid core coaxially integral with said needle element, and a compression spring seated on the inside of one end wall of the body of said solenoid valve assembly for biasing said movable core toward said main fuel jet, and temperature sensing means electrically connected between said solenoid coil and said power source and responsive to the engine temperature, said solenoid coil being energized when the engine temperature sensed by said temperature sensing means is below a predetermined valve, whereby said movable solenoid core is moved against the action of said compression spring to a position in which said needle element is unseated from said main fuel jet to provide increased communication between said float chamber and said main mixture circuitfor thereby causing said main mixture circuit to produce a rich airfuel mixture, and said solenoid coil being de-energized when the engine temperature sensed by said tempera ture sensing means is above the predetermined value, whereby said movable solenoid core is moved by the action of said compressions spring to a position in which said needle element is seated on said main fuel jet to provide communication between said float chamber and said main mixture circuit only through said conduit of said needle element for thereby causing a predetermined amount of liquid fuel to flow from said float chamber into said main mixture circuit to provide a lean air-fuel mixture.

2. In a carburetor for an internal combustion engine having a float chamber, a carburetor induction passage, a throttle valve disposed in said carburetor induction passage, a main venturi, and a small venturi, the combination comprising a main mixture circuit opening into said small venturi for supplying an air-fuel mixture to said induction passage for high-speed andacceleration operations of said engine, said main mixture circuit having'a main fuel jet communicating with said float chamber, a main fuel and air mixer communication with said main fuel jet and a main nozzle opening into said small venturi, said main fuel and air mixer having formed at its bottom an orifice and its top an air bleed vented from the atmosphere, the effective areas of said orifice and said air bleed being so calibrated as to provide a desired amount of lean air-fuel mixture so that a lean mixture is produced in said main mixture circuit for delivering to said engine, an idling and slow-running mixture circuit opening into said induction passage downstream of said main venturi for supplying an airfuel mixture to said induction passage for low-speed and deceleration operations of said engine, said idling and slow-running mixture circuit having a slow-running port located at a position closely adjacent to the periphery of said throttle valve when it is substantially fully closed and an idling port located downstream of said throttle valve, said idling and slow-running mixture circuit also having a slow-running fuel and air mixer which has formed at its bottom an orifice and a first air bleed vented from the ambient atmosphere and a second air bleed downstream of said first air bleed, the effective areas of said first and second air bleeds being so calibrated as to admit air from the ambient atmosphere at desired flow rates to provide the air-fuel mixture for the low-speed and deceleration operations of said engine, a solenoid valve assembly for controlling the effective area of said main fuel jet of said main mixture circuit, said solenoid valve assembly including a needle element which is axially movable into abutting engagement with said main fuel jet and which is provided at its operative end with a conical tip having a ally integral with said needle element and a compression spring seated on the inside of one end wall of the body of said solenoid valve assembly for biasing said movable core toward said main fuel jet, and a ther-' moswitch electrically connected between said power source and said solenoid coil, said thermoswitch being electrically connected to and actuated by a temperature detector adapted to detect the temperature of said en i e whereb when the e i e tem erat re se sed by s id tempera ture detector i iielow a pre etermi ned value, said thermoswitch is closed to energize said solenoid coil thereby to cause said movable solenoid core to move against the action of said compression spring to a position in which said needle element is unseated from said main fuel jet for providing increased comm unication between said float chamber and said main mixture circuit for thereby causing said main mixture circuit to produce a rich air-fuel mixture, and, when the engine temperature sensed by said temperature detector is beyond the predetermined value, said ther moswitch is opened to de-energize said solenoid coil thereby to cause said movable solenoid core to move by the action of said compression spring to a position in which said needle element is seated on said main fuel jet for providing communication between said float chamber and said main mixture circuit only through said conduit of said needle element for thereby causing a predetermined amount of liquid fuel to flow from said float chamber into said main mixture circuit to provide a lean air-fuel mixture. 

1. In a carburetor for an internal combustion engine having a float chamber, a carburetor induction passage, a throttle valve disposed in said carburetor induction passage, a main venturi, and a small venturi, the combination comprising a main mixture circuit opening into said small venturi for supplying an air-fuel mixture to said induction passage for high-speed and acceleration operations, of said engine, said main mixture circuit having a main fuel jet communicating with said float chamber, an idling and slow-running mixture circuit opening into said inDuction passage downstream of said main venturi for supplying an air-fuel mixture to said induction passage for low-speed and deceleration operations of said engines, said idling and slow-running mixture circuit having a slow-running port located at a position closely adjacent the periphery of said throttle valve when it is substantially fully closed and an idling port located downstream of said throttle valve, a solenoid valve assembly for controlling the effective area of said main fuel jet of said main mixture circuit, said solenoid valve assembly including a needle element which is axially movable into abutting engagement with said main fuel jet and which is provided at its operative end with a conical tip having a conduit extending therethrough, the effective area of said conduit being predetermined so that a liquid fuel may flow from said float chamber into said main mixture circuit when the conical tip of said needle element is in abutting engagement with the main fuel jet thereby to provide a satisfactory amount of lean mixture, said solenoid valve assembly further including a solenoid coil electrically connected in series with a power source, an axially movable solenoid core coaxially integral with said needle element, and a compression spring seated on the inside of one end wall of the body of said solenoid valve assembly for biasing said movable core toward said main fuel jet, and temperature sensing means electrically connected between said solenoid coil and said power source and responsive to the engine temperature, said solenoid coil being energized when the engine temperature sensed by said temperature sensing means is below a predetermined valve, whereby said movable solenoid core is moved against the action of said compression spring to a position in which said needle element is unseated from said main fuel jet to provide increased communication between said float chamber and said main mixture circuit for thereby causing said main mixture circuit to produce a rich air-fuel mixture, and said solenoid coil being deenergized when the engine temperature sensed by said temperature sensing means is above the predetermined value, whereby said movable solenoid core is moved by the action of said compressions spring to a position in which said needle element is seated on said main fuel jet to provide communication between said float chamber and said main mixture circuit only through said conduit of said needle element for thereby causing a predetermined amount of liquid fuel to flow from said float chamber into said main mixture circuit to provide a lean air-fuel mixture.
 2. In a carburetor for an internal combustion engine having a float chamber, a carburetor induction passage, a throttle valve disposed in said carburetor induction passage, a main venturi, and a small venturi, the combination comprising a main mixture circuit opening into said small venturi for supplying an air-fuel mixture to said induction passage for high-speed and acceleration operations of said engine, said main mixture circuit having a main fuel jet communicating with said float chamber, a main fuel and air mixer communication with said main fuel jet and a main nozzle opening into said small venturi, said main fuel and air mixer having formed at its bottom an orifice and its top an air bleed vented from the atmosphere, the effective areas of said orifice and said air bleed being so calibrated as to provide a desired amount of lean air-fuel mixture so that a lean mixture is produced in said main mixture circuit for delivering to said engine, an idling and slow-running mixture circuit opening into said induction passage downstream of said main venturi for supplying an air-fuel mixture to said induction passage for low-speed and deceleration operations of said engine, said idling and slow-running mixture circuit having a slow-running port located at a position closely adjacent to the periphery of said throttle valve when it is substantially fully closed and an idling port located downstream oF said throttle valve, said idling and slow-running mixture circuit also having a slow-running fuel and air mixer which has formed at its bottom an orifice and a first air bleed vented from the ambient atmosphere and a second air bleed downstream of said first air bleed, the effective areas of said first and second air bleeds being so calibrated as to admit air from the ambient atmosphere at desired flow rates to provide the air-fuel mixture for the low-speed and deceleration operations of said engine, a solenoid valve assembly for controlling the effective area of said main fuel jet of said main mixture circuit, said solenoid valve assembly including a needle element which is axially movable into abutting engagement with said main fuel jet and which is provided at its operative end with a conical tip having a conduit extending therethrough, the effective area of said conduit being predetermined so that liquid fuel may flow from said float chamber into said main mixture circuit when the conical tip of said needle element is in abutting engagement with said main fuel jet thereby to provide a satisfactory amount of lean mixture, said solenoid valve assembly further including a solenoid coil electrically connected in series with a power source, an axially movable solenoid core coaxially integral with said needle element and a compression spring seated on the inside of one end wall of the body of said solenoid valve assembly for biasing said movable core toward said main fuel jet, and a thermoswitch electrically connected between said power source and said solenoid coil, said thermoswitch being electrically connected to and actuated by a temperature detector adapted to detect the temperature of said engine, whereby when the engine temperature sensed by said temperature detector is below a predetermined value, said thermoswitch is closed to energize said solenoid coil thereby to cause said movable solenoid core to move against the action of said compression spring to a position in which said needle element is unseated from said main fuel jet for providing increased communication between said float chamber and said main mixture circuit for thereby causing said main mixture circuit to produce a rich air-fuel mixture, and, when the engine temperature sensed by said temperature detector is beyond the predetermined value, said thermoswitch is opened to de-energize said solenoid coil thereby to cause said movable solenoid core to move by the action of said compression spring to a position in which said needle element is seated on said main fuel jet for providing communication between said float chamber and said main mixture circuit only through said conduit of said needle element for thereby causing a predetermined amount of liquid fuel to flow from said float chamber into said main mixture circuit to provide a lean air-fuel mixture. 